Operational Training Units (OTU) and Replacement Training Units (RTU) were training organizations of the United States Army Air Forces during World War II. Unlike the schools of the Army Air Forces Training Command (AAFTC), OTU-RTU units were operational units of the four domestic numbered air forces along with I Troop Carrier Command and Air Transport Command, with the mission of final phase training new pilots or crews. Most were disbanded in the spring of 1944 and replaced by combat crew replacement centers assigned to base units.

When the Army Air Corps began its great expansion program in 1939, no provision for operational training existed outside the combat groups themselves. Graduates of the flying schools were assigned either to fill the requirements of existing combat squadrons or to round out the cadre taken from an older unit to form a new one. Each combat squadron was responsible for training its own personnel in order to meet proficiency standards set by training directives from the GHQ Air Force. This method was developed after World War I, and was used successfully in the peacetime Air Corps of the 1920s and 1930s.

With war imminent, the number of authorized groups in the Air Corps had risen from twenty-five in April 1939 to eighty-four. With the withdrawal of cadres to form new units for this expansion, the average level of experience in all groups had declined sharply, with a corresponding effect on operational training. Although there were multiple other causes for the inefficacy of training, such as a shortage of planes and maintenance services, it was clear enough to the Air Corps that it could not plan its training based upon having enough cadres sufficiently experienced to guarantee prompt lifting of whole units to the desired level of proficiency.

Major Robert B. Williams, an Air Corps observer in England between September 1940 and January 1941 who later commanded the Second Air Force and a bombardment division of the Eighth, reported favorably to the Office of the Chief of Air Corps on the merits of the Royal Air Force's operational training unit (OTU) system. The RAF program inspired a proposal in January 1942 by Brig. Gen. Follett Bradley, commanding III Bomber Command, to his superior at Third Air Force that an OTU system be instituted in the Air Force Combat Command (which had the responsibility for training new AAF units) as a means of having sufficient experienced personnel to train newly activated groups while not degrading the proficiency of groups headed for combat. Under the system that had been in place since 1939, so many experienced groups would likely be needed for immediate requirements overseas that a critical shortage of experienced personnel needed to develop new units was inevitable. Adapting from the existing system, Bradley proposed that certain groups be designated "parent" groups, with an authorized overstrength, to provide cadres for newly activated (or "satellite") groups and assume responsibility for their organization and training. Fresh graduates of training schools would bring the satellite groups to authorized strength and, in a constantly recurring pattern, also restore the parent group to its overstrength.

The plan was largely adopted by AFCC in February 1942 to direct operational training in the Second and Third Air Forces, where the bulk of new units were being activated, but by May the AFCC had been dissolved as an extraneous echelon of command and the OTU system was extended by Headquarters AAF to the First and Fourth Air Forces for training new fighter groups. In its first year the system proved difficult to implement to full effect. Unforeseen emergency demands from the combat theaters for experienced units, an erratic supply of combat-type aircraft for training purposes, and an uneven flow of personnel from the individual training programs produced uneven results that took time and experience to work out. By early 1943, however, the plan was in general operation and became increasingly effective in preparing combat groups for action.

In most instances, six months were required after the formation of a group to complete its organization and training. Operational training officially commenced for a new group on the day that the cadre was assigned it from the parent group's overstrength. In 1942 the four continental air forces provided training directives to familiarize key members of cadre with their obligations but these varied from air force to air force. Beginning in 1943, cadre leaders received standardized training through a thirty-day course of instruction at the Army Air Forces School of Applied Tactics (AAFSAT) in Orlando, Florida, established in November 1942 partly for this purpose. Cadres for medium and heavy bombardment units were trained by AAFSAT's Bombardment Department, fighter cadres by its Air Defense Department, and light bombardment cadres by the Air Support Department. The AAFSAT course consisted of both an academic and a practical phase. During the academic phase, cadre leaders reviewed problems of command, operations, and intelligence under expert tutelage related to the mission of their group. During the practical phase, the cadre was assigned to an AAFSAT base for 50 hours of operational exercises assisted by the base's demonstration squadron, including simulated combat missions. While practical experience was of value in preparing the cadres, close coordination of the academic and practical phases was sometimes lacking and the exercises also suffered occasionally from a lack of needed equipment. On returning to their assigned OTU stations, the cadres began training with their units, which by this time had usually reached regulation strength. Group instruction was divided in varying proportions between individual and team training but during the final phase both air and ground echelons functioned as nearly as possible as a self-contained combat unit.

While the OTU system was evolving as the most suitable means of training new groups for combat, a plan calling for the establishment of Replacement Training Units (RTUs) as a regular means of providing replacement crews and crew members was also being developed.

Until May 1942, when the RTU system was ordered into effect in the continental air forces, replacements for overseas units were procured by withdrawing qualified personnel from regular units stationed in the United States. This method, though simple, followed no orderly plan and jeopardized effective unit training by removing experienced personnel from U.S.-based groups. In order to establish a sounder method of providing combat replacements, AAF Headquarters directed that certain additional groups be listed as training organizations and maintained at an authorized over-strength to serve as reservoirs from which trained individuals and crews could be withdrawn for overseas shipment.